Alright, let's solve this math problem step by step. First, we have the number 768. Next, we see the multiplication sign, which means we need to multiply something. And what are we multiplying it by? We multiply it by 21. So, we're going to take the number 768 and add it to itself 21 times.
Step 1: 0 + 768 = 768
Step 2: 768 + 768 = 1536
Step 3: 1536 + 768 = 2304
Step 4: 2304 + 768 = 3072
Step 5: 3072 + 768 = 3840
Step 6: 3840 + 768 = 4608
Step 7: 4608 + 768 = 5376
Step 8: 5376 + 768 = 6144
Step 9: 6144 + 768 = 6912
Step 10: 6912 + 768 = 7680
Step 11: 7680 + 768 = 8448
Step 12: 8448 + 768 = 9216
Step 13: 9216 + 768 = 9984
Step 14: 9984 + 768 = 10752
Step 15: 10752 + 768 = 11520
Step 16: 11520 + 768 = 12288
Step 17: 12288 + 768 = 13056
Step 18: 13056 + 768 = 13824
Step 19: 13824 + 768 = 14592
Step 20: 14592 + 768 = 15360
Step 21: 15360 + 768 = 16128

So, 768*21 = 16128